Virtual Suite: declarative-performance-observer

This virtual test suite runs Web Platform Tests under wpt_internal/declarative-performance-observer with the --enable-features=DeclarativePerformanceObserver command-line argument to verify the Declarative Performance Observer feature.

For more details, see the design document: https://docs.google.com/document/d/1qdwkyXTYYUru5z8h4y1qBDgE7gWeWzg1VPEx4chdj5c/edit?usp=sharing